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State health officials expressing caution as tick-borne disease on the rise in Minnesota

A growing number of ticks in Minnesota are carrying organisms that cause disease in humans, raising the risk that a bite by a black-legged, or deer, tick might lead to illness.

From 2005 to 2008, staff with the Minnesota Department of Health collected ticks from regions of Minnesota where Lyme disease and other tick-borne diseases are commonly reported. Testing found that about one of every three adult black-legged ticks was positive for the Lyme disease bacteria.

Among ticks still in the nymphal stage, just over 10 percent were positive for the Lyme disease bacteria.

Nearly10 percent of the adult ticks and 5 percent of the immature ticks that were studied also tested positive for the organisms that cause human anaplasmosis and babesiosis.

Bites from black-legged ticks have led to record numbers of tick-borne disease cases in Minnesota in recent years. Between 1999 and 2003, around 50 cases of Lyme disease were reported annually to the state Health Department. But starting in 2004, the reported incidence has risen to about 1,000 cases a year.

The rate of human anaplasmosis and babesiosis also has risen. There were about 300 cases of anaplasmosis and 25 cases of babesiosis in 2007 and 2008.

Black-legged ticks are most common in hardwood forests of east central, north central and southeastern Minnesota. They have been expanding their range, however, and can now also be found in forested parts of west central, northwestern and northeastern Minnesota. The ticks also are common in wooded areas of Wisconsin and northeastern states. The ticks are most active from April through October. Mid-May through mid-July is the time of greatest activity for the nymph stage of the tick and therefore the highest-risk time for transmission of tick-borne disease to humans.

Immature black-legged ticks are especially tiny, about the size of a poppy seed. Because of their small size, many people don't notice if one of the ticks is attached to their skin, state health officials said.

"People who live in or visit wooded or brushy areas need to take precautions against tick bites," said Dr. Ruth Lynfield, state epidemiologist. "The rising number of disease cases suggests that Minnesota forests may contain greater numbers of infected ticks than in the past. It also suggests that too few Minnesotans are taking simple precautions to protect themselves."

Precautions are most important during the late spring, early summer and fall, when black-legged ticks are active. When spending time in wooded or brushy areas, use tick repellent containing DEET or permethrin. Repellents containing up to 30 percent DEET can safely be used on skin or clothing. Permethrin-based products, which are only applied to clothing, are highly effective and can last through several washings.

Since ticks climb up from the ground, focus repellent use below the knees. Also, wear long pants and light-colored clothing and walk in the center of trails.

After returning from the woods, check your body carefully for ticks and promptly remove any that are found. Black-legged ticks are smaller and darker than the common wood ticks that people may encounter at this time of year. They also lack the wood tick's characteristic white markings, and the back end of the female black-legged tick is reddish-orange in appearance.

Black-legged ticks need to be attached for 24 to 48 hours to transmit Lyme disease bacteria and 12 to 24 hours to transmit human anaplasmosis bacteria.

Symptoms of Lyme disease can include an expanding rash, fever, headache, chills, muscle pain, joint pain and fatigue. The rash, one of the earliest symptoms, typically appears between three and 30 days after an infectious tick bite. Not everyone with Lyme disease develops a rash, however.

Symptoms of human anaplasmosis and babesiosis include a high fever, chills, headache and muscle aches. With anaplasmosis, these symptoms appear within approximately one to three weeks after being bitten by an infected ticks. For babesiosis, the symptoms can appear in one to six weeks or more.

People who develop symptoms of a tick-related illness after spending time in a wooded area should see a physician promptly, even if they don't remember getting a tick bite. Early diagnosis and treatment are important in preventing severe illness.